Prospects for high-z cluster detections with Planck, based on a follow-up of 28 candidates using MegaCam at CFHT^⋆

¹ Laboratoire AIM, IRFU/Service d’Astrophysique − CEA/DSM − CNRS − Université Paris Diderot, Bât. 709, Saclay, 91191 Gif-sur-Yvette Cedex, France
e-mail: remco.van-der-burg@cea.fr
² DSM/IRFU/SPP, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
³ Institut d’Astrophysique Spatiale, CNRS (UMR 8617) Université Paris-Sud 11, Bâtiment 121, 91405 Orsay Cedex, France
⁴ Instituto de Astrofísica de Canarias, C/vía Láctea s/n, 38205, La Laguna, Tenerife, Spain
⁵ Dpto. Astrofísica, Universidad de La Laguna (ULL), 38206, La Laguna, Tenerife, Spain
⁶ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
⁷ Departments of Physics, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15217, USA
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁹ Laboratoire de Physique Subatomique et de Cosmologie, Université Joseph Fourier Grenoble I, CNRS/IN2P3, Institut National Polytechnique de Grenoble, 53 rue des Martyrs, 38026 Grenoble Cedex, France
¹⁰ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse Cedex 4, France
¹¹ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France

Received: 3 September 2015
Accepted: 1 December 2015

Abstract

The Planck catalogue of SZ sources limits itself to a significance threshold of 4.5 to ensure a low contamination rate by false cluster candidates. This means that only the most massive clusters at redshift z> 0.5, and in particular z> 0.7, are expected to enter into the catalogue, with a large number of systems in that redshift regime being expected around and just below that threshold. In this paper, we follow-up a sample of SZ sources from the Planck SZ catalogues from 2013 and 2015. In the latter maps, we consider detections around and at lower significance than the threshold adopted by the Planck Collaboration. To keep the contamination rate low, our 28 candidates are chosen to have significant WISE detections, in combination with non-detections in SDSS/DSS, which effectively selects galaxy cluster candidates at redshifts z ≳ 0.5. By taking r- and z-band imaging with MegaCam at CFHT, we bridge the 4000 Å rest-frame break over a significant redshift range, thus allowing accurate redshift estimates of red-sequence cluster galaxies up to z ~ 0.8. After discussing the possibility that an overdensity of galaxies coincides -by chance- with a Planck SZ detection, we confirm that 16 of the candidates have likely optical counterparts to their SZ signals, 13 (6) of which have an estimated redshift z> 0.5 (z> 0.7). The richnesses of these systems are generally lower than expected given the halo masses estimated from the Planck maps. However, when we follow a simplistic model to correct for Eddington bias in the SZ halo mass proxy, the richnesses are consistent with a reference mass-richness relation established for clusters detected at higher significance. This illustrates the benefit of an optical follow-up, not only to obtain redshift estimates, but also to provide an independent mass proxy that is not based on the same data the clusters are detected with, and thus not subject to Eddington bias.